Tighter octal parsing broke some tests and were disabled in
https://golang.org/cl/5530051

Those tests were broken.  The CSS decoder was supposed to see CSS
hex escape sequences of the form '\' <hex>+, but those escape
sequences were instead being consumed by the template parser.

This change properly escapes those escape sequences, and uses
proper escaping for NULs.

R=golang-dev, rsc, nigeltao
CC=golang-dev
https://golang.org/cl/5529073